The present invention relates to apparatuses and methods for monitoring the health of a mechanical system. The invention is particularly, but not exclusively concerned with the monitoring of the health of a complex mechanical system, and especially power plant, including for example gas turbine, spark ignition and compression ignition internal combustion engines.
The health of a system can be considered a measure of the condition of a system against expected norms. A healthy system is one whose condition closely matches expectations, whereas an unhealthy system is one whose condition differs from what would be expected, indicating for example deterioration of, or a possible problem with the system. The ability to monitor the health of a system can therefore allow such deterioration and/or problems to be detected and, if necessary, addressed at an early stage.
For example, U.S. Pat. No. 5,684,718 describes a non-real time system for monitoring the operation of an electric generator in which vibration and load data are combined to produce a single signal which is then compared with stored data representative of maximum acceptable combinations of the two parameters. Essentially the system is an automated “look up table” which issues warnings when vibrations have exceeded acceptable limits.
In order to determine the condition, and consequently health, of a system, it is normal to monitor and analyse a series of measurable indicators which themselves reflect aspects of the condition of the system. For instance, taking the example of a gas turbine, one might monitor performance parameters such as turbine and compressor operating temperatures and pressures and spool speeds. To obtain a fuller overall picture of the engine's condition, these performance parameters can be supplemented with further condition indicators including, for example, vibration measurements and measurements of the particulate material entrained in the circulating oil.
Particularly with complex mechanical systems such as gas turbines, the number of indicators that must be monitored to obtain a useful overall picture of the system's condition can be high. This in turn means that the task of analysing the complete series of indicators to determine the health of the engine is a complex one, typically requiring a skilled expert to analyse the data off-line.
WO02/03041 describes methods for monitoring the health of a system such as a gas turbine engine using performance data and vibration data acquired from analogue vibration transducers connected to the engine. An embodiment of the method disclosed has been implemented in the QUICK™ system produced by Oxford Biosignals Ltd. of Oxford, UK.
Existing airborne/aircraft vibration monitoring (AVM) systems also use vibration data acquired from vibration transducers or accelerometers connected to the engine components which are desired to be monitored. Whilst such systems provide useful information from the vibration of the various components of the engines to which transducers are attached, they do not provide a general picture of the engine as a whole and are thus less capable of detecting unusual features such as fan flutter, bird ingestion, stall, ice shedding, etc.
Although it is acknowledged that a skilled test-bed engineer with many years' experience can often identify a fault with an engine by listening to the sounds produced by the engine, the use of a microphone to provide data for engine health monitoring has not previously been seriously considered due to the perceived difficulties in obtaining useful information from a system that is inherently noisy. Furthermore, in the case of gas turbine engines, in order to receive information from the engine, such a microphone would have to be mounted in an airflow that is travelling at several hundred miles per hour and would also be subject to effects from fan blade passing.